PORTSMOUTH — The Sarah Mildred Long Bridge connecting Portsmouth and Kittery, Maine, sustained structural damage Monday after a 473-foot tanker carrying thousands of metric tons of tallow oil, yellow grease and fuel oil drifted away from a state pier and slammed into the bridge broadside.

The bridge, which topped the state's list of bridges in need of critical repairs even before the accident, will remain closed to vehicular traffic until it can be thoroughly inspected and fixed, said Chris Clement, the state's transportation commissioner.

Clement said he didn't know how long the bridge will be closed, but indicated that it could be several days."I think that's a distinct possibility," he said of the bridge, which could be replaced beginning in 2014.

A preliminary inspection showed that a concrete sidewalk and two vertical support beams were heavily damaged, Clement said, but a closer inspection will begin this morning to determine the full extent of the structural repairs needed. Gov. Maggie Hassan plans to visit the bridge today.

River traffic was also shut down for several hours Monday afternoon, but resumed as soon as the MV Harbour Feature, a tanker registered in Portugal, was pulled away from the bridge, which carries traffic from the Route 1 Bypass over the Piscataqua River, and floated back to the pier with help from tugboats.

The bridge's closure means motorists have only one route across the river — the Interstate 95 bridge. The Memorial Bridge over Route 1 is being replaced and is slated to open this summer.

The Coast Guard is leading the investigation into the accident, which occurred shortly before 1:30 p.m. when the vessel broke loose from its mooring and drifted about 30 yards from the New Hampshire Port Authority's Market Street Marine Terminal. It struck the bridge and remained against it until it was moved back to the state pier around 6 p.m. Coast Guard Lt. Nick Barrow said the tanker was carrying 11,500 metric tons of tallow oil and 3,600 metric tons of yellow grease as cargo. It also had 2,019 metric tons of its own fuel oil and 54 metric tons of diesel on board.

Barrow said it didn't appear that any of the substances leaked into the river, but environmental agencies were on hand just in case.

The vessel sustained a six- to 12-inch gash near a port ballast tank about three feet above the water line. Large dents were also visible on the rear corner of the ship. A 20-foot-long area of dents and scrapes was also seen on one side of the ship.

Barrow said there was a crew of about 20 on board. No one was hurt.

The vessel will have to undergo several inspections to determine its structural integrity, including an underwater survey by divers, before it's allowed to sail again.

Geno Marconi, port director, said the ship arrived at the Market Street Marine Terminal after stopping at the Sprague Energy port upriver. It planned to fuel its own engine, but never had a chance before it drifted into the bridge, he said.

At this point, Barrow said it's not known why the vessel drifted away.

Marconi said that there was "no failure" at the port facility. "Our mooring equipment didn't break," he said.

While the ship was secured to the dock, "something happened on board the ship," Marconi said, adding that the lines used to secure the vessel belong to the ship.

Grant Brown, vice president of investor relations and marketing at Sprague Energy, said the ship made a stop at its River Road facility Monday morning and then left around 11:15 a.m.

"There was nothing that suggested that there were any problems," Brown said.

According to Marconi, the vessel is owned by Italy-based Zacchello Group and managed by Nordic Tankers of Denmark.